Uncoupling of astrogliosis from epileptogenesis in adenosine kinase (ADK) transgenic mice

Astrocyte mitochondria: Potential therapeutic targets for epilepsy

Epilepsy is a chronic, relapsing neurological disorder, and current treatments focus primarily on neurons, yet one-third of patients still develop drug-resistant epilepsy. Therefore, there is an urgent need to explore new therapeutic targets. Interestingly, astrocytes can transfer their healthy mitochondria into neighboring neurons, thus preventing neuronal damage. Astrocyte mitochondria have been shown to have a therapeutic role in stroke and neurodegenerative diseases. However, their therapeutic effect in epilepsy and its related mechanisms have been less studied. In this review, we mainly summarize the regulatory role of astrocyte mitochondria in glutamate, calcium ion, and adenosine triphosphate (ATP) homeostasis and outline the protective role of astrocyte mitochondria in nervous system diseases, revealing a new target for epilepsy treatment.

Developmental and foliation changes due to dysregulation of adenosine kinase in the cerebellum

Adenosine kinase (ADK), the major adenosine-metabolizing enzyme, plays a key role in brain development and disease. In humans, mutations in the Adk gene have been linked to developmental delay, stunted growth, and intellectual disability. To better understand the role of ADK in brain development, it is important to dissect the specific roles of the two isoforms of the enzyme expressed in the cytoplasm (ADK-S) and cell nucleus (ADK-L). We, therefore, studied brain development in Adk-tg transgenic mice, which only express ADK-S in the absence of ADK-L throughout development. In the mutant animals, we found a reduction in the overall brain, body size, and weight during fetal and postnatal development. As a major developmental abnormality, we found a profound change in the foliation pattern of the cerebellum. Strikingly, our results indicated aberrant Purkinje cells arborization at P9 and accelerated cell death at P6 and P9. We found defects in cerebellar cell proliferation and migration using a bromodeoxyuridine (BrdU)-based cell proliferation assay at postnatal day 7. Our data demonstrate that dysregulation of ADK expression during brain development profoundly affects brain growth and differentiation.

Astrocyte-neuron circuits in epilepsy

The epilepsies are a diverse spectrum of disease states characterized by spontaneous seizures and associated comorbidities. Neuron-focused perspectives have yielded an array of widely used anti-seizure medications and are able to explain some, but not all, of the imbalance of excitation and inhibition which manifests itself as spontaneous seizures. Furthermore, the rate of pharmacoresistant epilepsy remains high despite the regular approval of novel anti-seizure medications. Gaining a more complete understanding of the processes that turn a healthy brain into an epileptic brain (epileptogenesis) as well as the processes which generate individual seizures (ictogenesis) may necessitate broadening our focus to other cell types. As will be detailed in this review, astrocytes augment neuronal activity at the level of individual neurons in the form of gliotransmission and the tripartite synapse. Under normal conditions, astrocytes are essential to the maintenance of blood-brain barrier integrity and remediation of inflammation and oxidative stress, but in epilepsy these functions are impaired. Epilepsy results in disruptions in the way astrocytes relate to each other by gap junctions which has important implications for ion and water homeostasis. In their activated state, astrocytes contribute to imbalances in neuronal excitability due to their decreased capacity to take up and metabolize glutamate and an increased capacity to metabolize adenosine. Furthermore, due to their increased adenosine metabolism, activated astrocytes may contribute to DNA hypermethylation and other epigenetic changes that underly epileptogenesis. Lastly, we will explore the potential explanatory power of these changes in astrocyte function in detail in the specific context of the comorbid occurrence of epilepsy and Alzheimer's disease and the disruption in sleep-wake regulation associated with both conditions.

Deep brain stimulation of the anterior thalamus attenuates PTZ kindling with concomitant reduction of adenosine kinase expression in rats

BACKGROUND

Deep brain stimulation (DBS) of the anterior nucleus of the thalamus (ANT) is an emerging therapy to provide seizure control in patients with refractory epilepsy, although its therapeutic mechanisms remain elusive.

OBJECTIVE

We tested the hypothesis that ANT-DBS might interfere with the kindling process using three experimental groups: PTZ, DBS-ON and DBS-OFF.

METHODS

79 male rats were used in two experiments and exposed to chemical kindling with pentylenetetrazole (PTZ, 30 mg/kg i.p.), delivered three times a week for a total of 18 kindling days (KD). These animals were divided into two sets of three groups: PTZ (n = 26), DBS-ON (n = 28) and DBS-OFF (n = 25). ANT-DBS (130 Hz, 90 μs, and 200 μA) was paired with PTZ injections, while DBS-OFF group, although implanted remained unstimulated. After KD 18, the first set of PTZ-treated animals and an additional group of 11 naïve rats were euthanized for brain extraction to study adenosine kinase (ADK) expression. To observe possible long-lasting effects of ANT stimulation, the second set of animals underwent a 1-week treatment and stimulation-free period after KD 18 before a final PTZ challenge.

RESULTS

ANT-DBS markedly attenuated kindling progression in the DBS-ON group, which developed seizure scores of 2.4 on KD 13, whereas equivalent seizure scores were reached in the DBS-OFF and PTZ groups as early as KD5 and KD6, respectively. The incidence of animals with generalized seizures following 3 consecutive PTZ injections was 94%, 74% and 21% in PTZ, DBS-OFF and DBS-ON groups, respectively. Seizure scores triggered by a PTZ challenge one week after cessation of stimulation revealed lasting suppression of seizure scores in the DBS-ON group (2.7 ± 0.2) compared to scores of 4.5 ± 0.1 for the PTZ group and 4.3 ± 0.1 for the DBS-OFF group (P = 0.0001). While ANT-DBS protected hippocampal cells, the expression of ADK was decreased in the DBS-ON group compared to both PTZ (P < 0.01) and naïve animals (P < 0.01).

CONCLUSIONS

Our study demonstrates that ANT-DBS interferes with the kindling process and reduced seizure activity was maintained after a stimulation free period of one week. Our findings suggest that ANT-DBS might have additional therapeutic benefits to attenuate seizure progression in epilepsy.

Genetic variations of adenosine kinase as predictable biomarkers of efficacy of vagus nerve stimulation in patients with pharmacoresistant epilepsy

OBJECTIVE

Vagus nerve stimulation (VNS) is an alternative treatment option for individuals with refractory epilepsy, with nearly 40% of patients showing no benefit after VNS and only 6%-8% achieving seizure freedom. It is presently unclear why some patients respond to treatment and others do not. Therefore, identification of biomarkers to predict efficacy of VNS is of utmost importance. The objective of this study was to explore whether genetic variations in genes involved in adenosine kinase (ADK), ecto-5'-nucleotidase (NT5E), and adenosine A1 receptor (A1R) are linked to outcome of VNS in patients with refractory epilepsy.

METHODS

Thirty single-nucleotide polymorphisms (SNPs), including 9 in genes encoding ADK, 3 in genes encoding NT5E, and 18 in genes encoding A1R, were genotyped in 194 refractory epilepsy patients who underwent VNS. The chi-square test and binary logistic regression were used to determine associations between genetic differences and VNS efficacy.

RESULTS

A significant association between ADK SNPs rs11001109, rs7899674, and rs946185 and seizure reduction with VNS was found. Regardless of sex, age, seizure frequency and type, antiseizure drug use, etiology, and prior surgical history, all patients (10/10 patients [100%]) with minor allele homozygosity at rs11001109 (genotype AA) or rs946185 (AA) achieved > 50% seizure reduction and 4 patients (4/10 [40%]) achieved seizure freedom. VNS therapy demonstrated higher efficacy among carriers of minor allele rs7899674 (CG + GG) (68.3% vs 48.8% for patients with major allele homozygosity).

CONCLUSIONS

Homozygous ADK SNPs rs11001109 (AA) and rs946185 (AA), as well as minor allele rs7899674 (CG + GG), may serve as useful biomarkers for prediction of VNS therapy outcome.

Rasmussen's encephalitis: mechanisms update and potential therapy target

Rasmussen’s encephalitis (RE) is rare neurological diseases characterized as epilepsia partialis continua, invariably hemiparesis, and cognitive impairment. This disease is encountered frequently in childhood and presents with progressive atrophy of the unilateral hemisphere, and there are also sustained neurological complications. Owing to uncertain pathogenesis, the most effective way to limit the influence of seizures currently is cerebral hemispherectomy. In this review, we focus on four main lines of pathogenesis: virus infection, antibody-mediated, cell-mediated immunity, and microglia activation. Although one or more antigenic epitopes may give rise to infiltrating T cell responses in RE brain tissue, no exact antigen was confirmed as the definite cause of the disease. On the other hand, the appearance of antibodies related with RE seem to be a secondary pathological process. Synthetic studies have suggested an adaptive immune mechanism mediated by CD8⁺ T cells and an innate immune mechanism mediated by activated microglia and neuroglia. Accordingly, opinions have been raised that immunomodulatory treatments aimed at initial damage to the brain that are induced by cytotoxic CD8⁺ T cell lymphocytes and microglia in the early stage of RE slow down disease progression. However, systematic exploration of the theory behind these therapeutic effects based on multicenter and large sample studies are needed. In addition, dysfunction of the adenosine system, including the main adenosine removing enzyme adenosine kinase and adenosine receptors, has been demonstrated in RE, which might provide a novel therapeutic target for treatment of RE in future.

Brain somatic mutations in MTOR reveal translational dysregulations underlying intractable focal epilepsy

Brain somatic mutations confer genomic diversity in the human brain and cause neurodevelopmental disorders. Recently, brain somatic activating mutations in MTOR have been identified as a major etiology of intractable epilepsy in patients with cortical malformations. However, the molecular genetic mechanism of how brain somatic mutations in MTOR cause intractable epilepsy has remained elusive. In this study, translational profiling of intractable epilepsy mouse models with brain somatic mutations and genome-edited cells revealed a novel translational dysregulation mechanism and mTOR activation-sensitive targets mediated by human MTOR mutations that lead to intractable epilepsy with cortical malformation. These mTOR targets were found to be regulated by novel mTOR-responsive 5'-UTR motifs, distinct from known mTOR inhibition-sensitive targets regulated by 5' terminal oligopyrimidine motifs. Novel mTOR target genes were validated in patient brain tissues, and the mTOR downstream effector eIF4E was identified as a new therapeutic target in intractable epilepsy via pharmacological or genetic inhibition. We show that metformin, an FDA-approved eIF4E inhibitor, suppresses intractable epilepsy. Altogether, the present study describes translational dysregulation resulting from brain somatic mutations in MTOR, as well as the pathogenesis and potential therapeutic targets of intractable epilepsy.

Role of Adenosine in Epilepsy and Seizures

Adenosine is an endogenous anticonvulsant and neuroprotectant of the brain. Seizure activity produces large quantities of adenosine, and it is this seizure-induced adenosine surge that normally stops a seizure. However, within the context of epilepsy, adenosine plays a wide spectrum of different roles. It not only controls seizures (ictogenesis), but also plays a major role in processes that turn a normal brain into an epileptic brain (epileptogenesis). It is involved in the control of abnormal synaptic plasticity and neurodegeneration and plays a major role in the expression of comorbid symptoms and complications of epilepsy, such as sudden unexpected death in epilepsy (SUDEP). Given the important role of adenosine in epilepsy, therapeutic strategies are in development with the goal to utilize adenosine augmentation not only for the suppression of seizures but also for disease modification and epilepsy prevention, as well as strategies to block adenosine A_2A receptor overfunction associated with neurodegeneration. This review provides a comprehensive overview of the role of adenosine in epilepsy.

Transient use of a systemic adenosine kinase inhibitor attenuates epilepsy development in mice

OBJECTIVE

Over one-third of all patients with epilepsy are refractory to treatment and there is an urgent need to develop new drugs that can prevent the development and progression of epilepsy. Epileptogenesis is characterized by distinct histopathologic and biochemical changes, which include astrogliosis and increased expression of the adenosine-metabolizing enzyme adenosine kinase (ADK; EC 2.7.1.20). Increased expression of ADK contributes to epileptogenesis and is therefore a target for therapeutic intervention. We tested the prediction that the transient use of an ADK inhibitor administered during the latent phase of epileptogenesis can mitigate the development of epilepsy.

METHODS

We used the intrahippocampal kainic acid (KA) mouse model of temporal lobe epilepsy, which is characterized by ipsilateral hippocampal sclerosis with granule cell dispersion and the development of recurrent hippocampal paroxysmal discharges (HPDs). KA-injected mice were treated with the ADK inhibitor 5-iodotubercidin (5-ITU, 1.6 mg/kg, b.i.d., i.p.) during the latent phase of epileptogenesis from day 3-8 after injury; the period when gradual increases in hippocampal ADK expression begin to manifest. HPDs were assessed at 6 and 9 weeks after KA administration followed by epilepsy histopathology including assessment of granule cell dispersion, astrogliosis, and ADK expression.

RESULTS

5-ITU significantly reduced the percent time in seizures by at least 80% in 56% of mice at 6 weeks post-KA. This reduction in seizure activity was maintained in 40% of 5-ITU-treated mice at 9 weeks. 5-ITU also suppressed granule cell dispersion and prevented maladaptive ADK increases in these protected mice.

SIGNIFICANCE

Our results show that the transient use of a small-molecule ADK inhibitor, given during the early stages of epileptogenesis, has antiepileptogenic disease-modifying properties, which provides the rationale for further investigation into the development of a novel class of antiepileptogenic ADK inhibitors with increased efficacy for epilepsy prevention.

General Pathophysiology of Astroglia

Astroglial cells are involved in most if not in all pathologies of the brain. These cells can change the morpho-functional properties in response to pathology or innate changes of these cells can lead to pathologies. Overall pathological changes in astroglia are complex and diverse and often vary with different disease stages. We classify astrogliopathologies into reactive astrogliosis, astrodegeneration with astroglial atrophy and loss of function, and pathological remodelling of astrocytes. Such changes can occur in neurological, neurodevelopmental, metabolic and psychiatric disorders as well as in infection and toxic insults. Mutation in astrocyte-specific genes leads to specific pathologies, such as Alexander disease, which is a leukodystrophy. We discuss changes in astroglia in the pathological context and identify some molecular entities underlying pathology. These entities within astroglia may repent targets for novel therapeutic intervention in the management of brain pathologies.

A rapid and sensitive colorimetric assay for the determination of adenosine kinase activity

Adenosine kinase (ADK) plays an important role in the growth and development of organisms. A convenient, quick, reliable, sensitive and low-cost assay for ADK activity is of great significance. Here, we found the reaction system with bromothymol blue as the pH indicator had a maximum absorption peak at 614 nm. The absorbance change in 614 nm was positively correlated with the generated hydrogen ions in the reaction catalyzed by ADK. Then, we demonstrated this assay was feasible for ADK activity. Further, we analyzed the effects of buffer, bromothymol blue concentrations on the sensitivity of the assay, and investigated the sensitivity of ADK contents and adenosine concentration on the assay. Finally, we calculated the K_m and Vmax of ADK from Bombyx mori with this assay. Our results suggested this assay was quick, convenient, reliable, sensitive and economic for the activity of ADK. It is an excellent alternative for the conventional ADK assays.

Translational potential of astrocytes in brain disorders

Fundamentally, all brain disorders can be broadly defined as the homeostatic failure of this organ. As the brain is composed of many different cells types, including but not limited to neurons and glia, it is only logical that all the cell types/constituents could play a role in health and disease. Yet, for a long time the sole conceptualization of brain pathology was focused on the well-being of neurons. Here, we challenge this neuron-centric view and present neuroglia as a key element in neuropathology, a process that has a toll on astrocytes, which undergo complex morpho-functional changes that can in turn affect the course of the disorder. Such changes can be grossly identified as reactivity, atrophy with loss of function and pathological remodeling. We outline the pathogenic potential of astrocytes in variety of disorders, ranging from neurotrauma, infection, toxic damage, stroke, epilepsy, neurodevelopmental, neurodegenerative and psychiatric disorders, Alexander disease to neoplastic changes seen in gliomas. We hope that in near future we would witness glial-based translational medicine with generation of deliverables for the containment and cure of disorders. We point out that such as a task will require a holistic and multi-disciplinary approach that will take in consideration the concerted operation of all the cell types in the brain.

Adenosine receptors and epilepsy: current evidence and future potential

Adenosine receptors are a powerful therapeutic target for regulating epileptic seizures. As a homeostatic bioenergetic network regulator, adenosine is perfectly suited to establish or restore an ongoing balance between excitation and inhibition, and its anticonvulsant efficacy is well established. There is evidence for the involvement of multiple adenosine receptor subtypes in epilepsy, but in particular the adenosine A1 receptor subtype can powerfully and bidirectionally regulate seizure activity. Mechanisms that regulate adenosine itself are increasingly appreciated as targets to thus influence receptor activity and seizure propensity. Taken together, established evidence for the powerful potential of adenosine-based epilepsy therapies and new strategies to influence receptor activity can combine to capitalize on this endogenous homeostatic neuromodulator.

Upregulation of adenosine kinase in Rasmussen encephalitis

Rasmussen encephalitis (RE) is a rare neurologic disorder of childhood characterized by unihemispheric inflammation, progressive neurologic deficits, and intractable focal epilepsy. The pathogenesis of RE is still enigmatic. Adenosine is a key endogenous signaling molecule with anticonvulsive and anti-inflammatory effects, and our previous work demonstrated that dysfunction of the adenosine kinase (ADK)–adenosine system and astrogliosis are the hallmarks of epilepsy. We hypothesized that the epileptogenic mechanisms underlying RE are related to changes in ADK expression and that those changes might be associated with the development of epilepsy in RE patients. Immunohistochemistry was used to examine the expression of ADK and glial fibrillary acidic protein in surgically resected human epileptic cortical specimens from RE patients (n = 12) and compared with control cortical tissues (n = 6). Adenosine kinase expression using Western blot and enzymatic activity for ADK were assessed in RE versus control samples. Focal astrogliosis and marked expression of ADK were observed in the lesions of RE. Significantly greater ADK expression in RE versus controls was demonstrated by Western blot, and greater enzymatic activity for ADK was demonstrated using an enzyme-coupled bioluminescent assay. These results suggest that upregulation of ADK is a common pathologic hallmark of RE and that ADK might be a target in the treatment of epilepsy associated with RE.

Glial adenosine kinase--a neuropathological marker of the epileptic brain

Experimental research over the past decade has supported the critical role of astrocytes activated by different types of injury and the pathophysiological processes that underlie the development of epilepsy. In both experimental and human epileptic tissues astrocytes undergo complex changes in their physiological properties, which can alter glio-neuronal communication, contributing to seizure precipitation and recurrence. In this context, understanding which of the molecular mechanisms are crucially involved in the regulation of glio-neuronal interactions under pathological conditions associated with seizure development is important to get more insight into the role of astrocytes in epilepsy. This article reviews current knowledge regarding the role of glial adenosine kinase as a neuropathological marker of the epileptic brain. Both experimental findings in clinically relevant models, as well as observations in drug-resistant human epilepsies will be discussed, highlighting the link between astrogliosis, dysfunction of adenosine homeostasis and seizure generation and therefore suggesting new strategies for targeting astrocyte-mediated epileptogenesis.

Adenosine kinase: exploitation for therapeutic gain

Adenosine kinase (ADK; EC 2.7.1.20) is an evolutionarily conserved phosphotransferase that converts the purine ribonucleoside adenosine into 5'-adenosine-monophosphate. This enzymatic reaction plays a fundamental role in determining the tone of adenosine, which fulfills essential functions as a homeostatic and metabolic regulator in all living systems. Adenosine not only activates specific signaling pathways by activation of four types of adenosine receptors but it is also a primordial metabolite and regulator of biochemical enzyme reactions that couple to bioenergetic and epigenetic functions. By regulating adenosine, ADK can thus be identified as an upstream regulator of complex homeostatic and metabolic networks. Not surprisingly, ADK dysfunction is involved in several pathologies, including diabetes, epilepsy, and cancer. Consequently, ADK emerges as a rational therapeutic target, and adenosine-regulating drugs have been tested extensively. In recent attempts to improve specificity of treatment, localized therapies have been developed to augment adenosine signaling at sites of injury or pathology; those approaches include transplantation of stem cells with deletions of ADK or the use of gene therapy vectors to downregulate ADK expression. More recently, the first human mutations in ADK have been described, and novel findings suggest an unexpected role of ADK in a wider range of pathologies. ADK-regulating strategies thus represent innovative therapeutic opportunities to reconstruct network homeostasis in a multitude of conditions. This review will provide a comprehensive overview of the genetics, biochemistry, and pharmacology of ADK and will then focus on pathologies and therapeutic interventions. Challenges to translate ADK-based therapies into clinical use will be discussed critically.

Homeostatic control of synaptic activity by endogenous adenosine is mediated by adenosine kinase

Extracellular adenosine, a key regulator of neuronal excitability, is metabolized by astrocyte-based enzyme adenosine kinase (ADK). We hypothesized that ADK might be an upstream regulator of adenosine-based homeostatic brain functions by simultaneously affecting several downstream pathways. We therefore studied the relationship between ADK expression, levels of extracellular adenosine, synaptic transmission, intrinsic excitability, and brain-derived neurotrophic factor (BDNF)-dependent synaptic actions in transgenic mice underexpressing or overexpressing ADK. We demonstrate that ADK: 1) Critically influences the basal tone of adenosine, evaluated by microelectrode adenosine biosensors, and its release following stimulation; 2) determines the degree of tonic adenosine-dependent synaptic inhibition, which correlates with differential plasticity at hippocampal synapses with low release probability; 3) modulates the age-dependent effects of BDNF on hippocampal synaptic transmission, an action dependent upon co-activation of adenosine A2A receptors; and 4) influences GABAA receptor-mediated currents in CA3 pyramidal neurons. We conclude that ADK provides important upstream regulation of adenosine-based homeostatic function of the brain and that this mechanism is necessary and permissive to synaptic actions of adenosine acting on multiple pathways. These mechanistic studies support previous therapeutic studies and implicate ADK as a promising therapeutic target for upstream control of multiple neuronal signaling pathways crucial for a variety of neurological disorders.

The role of astroglia in the epileptic brain

Epilepsies comprise a family of multifactorial neurological disorders that affect at least 50 million people worldwide. Despite a long history of neurobiological and clinical studies the mechanisms that lead the brain network to a hyperexcitable state and to the intense, massive neuronal discharges reflecting a seizure episode are only partially defined. Most epilepsies of genetic origin are related to mutations in ionic channels that cause neuronal hyperexcitability. However, idiopathic epilepsies of unclear origin represent the majority of these brain disorders. A large body of evidence suggests that in the epileptic brain neurons are not the only players. Indeed, the glial cell astrocyte is known to be morphologically and functionally altered in different types of epilepsy. Although it is unclear whether these astrocyte dysfunctions can have a causative role in epileptogenesis, the hypothesis that astrocytes contribute to epileptiform activities recently received a considerable experimental support. Notably, currently used antiepileptic drugs, that act mainly on neuronal ion channels, are ineffective in a large group of patients. Clarifying astrocyte functions in the epileptic brain tissue could unveil astrocytes as novel therapeutic targets. In this review we present first a short overview on the role of astrocytes in the epileptic brain starting from the "historical" observations on their fundamental modulation of brain homeostasis, such as the control of water content, ionic equilibrium, and neurotransmitters concentrations. We then focus our review on most recent studies that hint at a distinct contribution of these cells in the generation of focal epileptiform activities.

Overexpression of ADK in human astrocytic tumors and peritumoral tissue is related to tumor-associated epilepsy

PURPOSE

Adenosine kinase (ADK), a largely astrocyte-based metabolic enzyme, regulates adenosine homeostasis in the brain. Overexpression of ADK decreases extracellular adenosine and consequently leads to seizures. We hypothesized that dysfunction in the metabolism of tumor astrocytes is related to changes in ADK expression and that those changes might be associated with the development of epilepsy in patients with tumors.

METHODS

We compared ADK expression and cellular distribution in surgically removed tumor tissue (n = 45) and peritumoral cortex (n = 20) of patients with glial and glioneuronal tumors to normal control tissue obtained at autopsy (n = 11). In addition, we compared ADK expression in tumor patients with and without epilepsy. To investigate ADK expression, we used immunohistochemistry and Western blot analysis. ADK activity measurement was performed in surgical specimens of astrocytomas World Health Organization (WHO) grade III (n = 3), peritumoral cortex (n = 3), and nonepileptic cortex (n = 3).

KEY FINDINGS

Immunohistochemistry predominantly showed cytoplasmic labeling in tumors and peritumoral tissue containing infiltrating tumor cells. ADK immunoreactivity was significantly stronger in tumor and peritumoral tissue compared to normal white matter and normal cortex, especially in astrocytoma WHO grade III, as confirmed by Western blot analysis and ADK activity measurements. Importantly, we found a significantly higher expression of ADK in the peritumoral infiltrated tissue of patients with epilepsy than in patients without epilepsy.

SIGNIFICANCE

These results suggest a dysregulation of ADK in astrocytic brain tumors. Moreover, the upregulation of ADK observed in peritumoral infiltrated tissue of glioma patients with epilepsy supports the role of this enzyme in tumor-associated epilepsy.

Local disruption of glial adenosine homeostasis in mice associates with focal electrographic seizures: a first step in epileptogenesis?

Astrogliosis and associated dysfunction of adenosine homeostasis are pathological hallmarks of the epileptic brain and thought to contribute to seizure generation in epilepsy. The authors hypothesized that astrogliosis-an early component of the epileptogenic cascade-might be linked to focal seizure onset. To isolate the contribution of astrogliosis to ictogenesis from other pathological events involved in epilepsy, the authors used a minimalistic model of epileptogenesis in mice, based on a focal onset status epilepticus triggered by intra-amygdaloid injection of kainic acid. The authors demonstrate acute neuronal cell loss restricted to the injected amygdala and ipsilateral CA3, followed 3 weeks later by focal astrogliosis and overexpression of the adenosine-metabolizing enzyme adenosine kinase (ADK). Using synchronous electroencephalographic recordings from multiple depth electrodes, the authors identify the KA-injected amygdala and ipsilateral CA3 as two independent foci for the initiation of non-synchronized electrographic subclinical seizures. Importantly, seizures remained focal and restricted to areas of ADK overexpression. However, after systemic application of a non-convulsive dose of an adenosine A(1) -receptor antagonist, seizures in amygdala and CA3 immediately synchronized and spread throughout the cortex, leading to convulsive seizures. This focal seizure phenotype remained stable over at least several weeks. We conclude that astrogliosis via disruption of adenosine homeostasis per se and in the absence of any other overt pathology, is associated with the emergence of spontaneous recurrent subclinical seizures, which remain stable over space and time. A secondary event, here mimicked by brain-wide disruption of adenosine signaling, is likely required to turn pre-existing subclinical seizures into a clinical seizure phenotype.

Purines and neuronal excitability: links to the ketogenic diet

ATP and adenosine are purines that play dual roles in cell metabolism and neuronal signaling. Acting at the A(1) receptor (A(1)R) subtype, adenosine acts directly on neurons to inhibit excitability and is a powerful endogenous neuroprotective and anticonvulsant molecule. Previous research showed an increase in ATP and other cell energy parameters when an animal is administered a ketogenic diet, an established metabolic therapy to reduce epileptic seizures, but the relationship among purines, neuronal excitability and the ketogenic diet was unclear. Recent work in vivo and in vitro tested the specific hypothesis that adenosine acting at A(1)Rs is a key mechanism underlying the success of ketogenic diet therapy and yielded direct evidence linking A(1)Rs to the antiepileptic effects of a ketogenic diet. Specifically, an in vitro mimic of a ketogenic diet revealed an A(1)R-dependent metabolic autocrine hyperpolarization of hippocampal neurons. In parallel, applying the ketogenic diet in vivo to transgenic mouse models with spontaneous electrographic seizures revealed that intact A(1)Rs are necessary for the seizure-suppressing effects of the diet. This is the first direct in vivo evidence linking A(1)Rs to the antiepileptic effects of a ketogenic diet. Other predictions of the relationship between purines and the ketogenic diet are discussed. Taken together, recent research on the role of purines may offer new opportunities for metabolic therapy and insight into its underlying mechanisms.

Upregulation of adenosine kinase in astrocytes in experimental and human temporal lobe epilepsy

PURPOSE

Adenosine kinase (ADK) represents the key metabolic enzyme for the regulation of extracellular adenosine levels in the brain. In adult brain, ADK is primarily present in astrocytes. Several lines of experimental evidence support a critical role of ADK in different types of brain injury associated with astrogliosis, which is also a prominent morphologic feature of temporal lobe epilepsy (TLE). We hypothesized that dysregulation of ADK is an ubiquitous pathologic hallmark of TLE.

METHODS

Using immunocytochemistry and Western blot analysis, we investigated ADK protein expression in a rat model of TLE during epileptogenesis and the chronic epileptic phase and compared those findings with tissue resected from TLE patients with mesial temporal sclerosis (MTS).

KEY FINDINGS

In rat control hippocampus and cortex, a low baseline expression of ADK was found with mainly nuclear localization. One week after the electrical induction of status epilepticus (SE), prominent up-regulation of ADK became evident in astrocytes with a characteristic cytoplasmic localization. This increase in ADK persisted at least for 3-4 months after SE in rats developing a progressive form of epilepsy. In line with the findings from the rat model, expression of astrocytic ADK was also found to be increased in the hippocampus and temporal cortex of patients with TLE. In addition, in vitro experiments in human astrocyte cultures showed that ADK expression was increased by several proinflammatory molecules (interleukin-1β and lipopolysaccharide).

SIGNIFICANCE

These results suggest that dysregulation of ADK in astrocytes is a common pathologic hallmark of TLE. Moreover, in vitro data suggest the existence of an additional layer of modulatory crosstalk between the astrocyte-based adenosine cycle and inflammation. Whether this interaction also can play a role in vivo needs to be further investigated.

Adenosine kinase as a target for therapeutic antisense strategies in epilepsy

PURPOSE

Given the high incidence of refractory epilepsy, novel therapeutic approaches and concepts are urgently needed. To date, viral-mediated delivery and endogenous expression of antisense sequences as a strategy to prevent seizures have received little attention in epilepsy therapy development efforts. Here we validate adenosine kinase (ADK), the astrocyte-based key negative regulator of the brain's endogenous anticonvulsant adenosine, as a potential therapeutic target for antisense-mediated seizure suppression.

METHODS

We developed adenoassociated virus 8 (AAV8)-based gene therapy vectors to selectively modulate ADK expression in astrocytes. Cell type selectivity was achieved by expressing an Adk-cDNA in sense or antisense orientation under the control of an astrocyte-specific gfaABC1D promoter. Viral vectors where injected into the CA3 of wild-type mice or spontaneously epileptic Adk-tg transgenic mice that overexpress ADK in brain. After virus injection, ADK expression was assessed histologically and biochemically. In addition, intracranial electroencephalography (EEG) recordings were obtained.

KEY FINDINGS

We demonstrate in wild-type mice that viral overexpression of ADK within astrocytes is sufficient to trigger spontaneous recurrent seizures in the absence of any other epileptogenic event, whereas ADK downregulation via AAV8-mediated RNA interference almost completely abolished spontaneous recurrent seizures in Adk-tg mice.

SIGNIFICANCE

Our data demonstrate that modulation of astrocytic ADK expression can trigger or prevent seizures, respectively. This is the first study to use an antisense approach to validate ADK as a rational therapeutic target for the treatment of epilepsy and suggests that gene therapies based on the knock down of ADK might be a feasible approach to control seizures in refractory epilepsy.

Methylxanthines, seizures, and excitotoxicity

Clinical evidence, in particular the wide use of theophylline as a bronchodilator, suggests that methylxanthines can cause seizures in patients without known underlying epilepsy. Theophylline is also known to be an added risk factor for seizure exacerbation in patients with epilepsy. The proconvulsant activity of methylxanthines can best be explained by their antagonizing the brain's own anticonvulsant adenosine. Recent evidence suggests that adenosine dysfunction is a pathological hallmark of epilepsy contributing to seizure generation and seizure spread. Conversely, adenosine augmentation therapies are effective in seizure suppression and prevention, whereas adenosine receptor antagonists such as methylxanthines generally exacerbate seizures. The impact of the methylxanthines caffeine and theophylline on seizures and excitotoxicity depends on timing, dose, and acute versus chronic use. New findings suggest a role of free radicals in theophylline-induced seizures, and adenosine-independent mechanisms for seizure generation have been proposed.

Manipulation of adenosine kinase affects sleep regulation in mice

Sleep and sleep intensity are enhanced by adenosine and its receptor agonists, whereas adenosine receptor antagonists induce wakefulness. Adenosine kinase (ADK) is the primary enzyme metabolizing adenosine in adult brain. To investigate whether adenosine metabolism or clearance affects sleep, we recorded sleep in mice with engineered mutations in Adk. Adk-tg mice overexpress a transgene encoding the cytoplasmic isoform of ADK in the brain but lack the nuclear isoform of the enzyme. Wild-type mice and Adk(+/-) mice that have a 50% reduction of the cytoplasmic and the nuclear isoforms of ADK served as controls. Adk-tg mice showed a remarkable reduction of EEG power in low frequencies in all vigilance states and in theta activity (6.25-11 Hz) in rapid eye movement (REM) sleep and waking. Adk-tg mice were awake 58 min more per day than wild-type mice and spent significantly less time in REM sleep (102 ± 3 vs 128 ± 3 min in wild type). After sleep deprivation, slow-wave activity (0.75-4 Hz), the intensity component of non-rapid eye movement sleep, increased significantly less in Adk-tg mice and their slow-wave energy was reduced. In contrast, the vigilance states and EEG spectra of Adk(+/-) and wild-type mice did not differ. Our data suggest that overexpression of the cytoplasmic isoform of ADK is sufficient to alter sleep physiology. ADK might orchestrate neurotransmitter pathways involved in the generation of EEG oscillations and regulation of sleep.

Inhibitory RNA in epilepsy: research tools and therapeutic perspectives

Since its discovery a decade ago, RNA interference (RNAi) has been developed not only into powerful experimental tools but also into promising novel therapeutics. In contrast to conventional antiepileptic drugs (AEDs) that target specific proteins such as ion channels or receptors, RNAi-based therapeutics exploit an endogenous regulatory mechanism of gene expression and thereby are poised to prevent or reverse pathogenetic mechanisms involved in seizure development. Therapeutic RNAi has been widely explored for dominant targets involved in neurodegenerative diseases; however, their use for epilepsy therapy has received less attention. This review discusses potential RNAi-based targets that are of interest for epilepsy therapy, including adenosine kinase (ADK), the key negative regulator of the brain's endogenous anticonvulsant adenosine. Overexpression of ADK, and the resulting adenosine deficiency, are pathologic hallmarks of the sclerotic epileptic brain, and have been implicated in seizure generation. Therefore, RNAi-strategies aimed at reducing ADK (and increasing adenosine) are based on a direct neurochemical rationale that has recently been explored experimentally using ex vivo and in vivo gene therapy approaches. Technical issues and challenges remain before those promising tools can be developed into future therapeutics for epilepsy.

Astrocytes derived from fetal neural progenitor cells as a novel source for therapeutic adenosine delivery

PURPOSE

Intracerebral delivery of anti-epileptic compounds represents a novel strategy for the treatment of refractory epilepsy. Adenosine is a possible candidate for local delivery based on its proven anti-epileptic effects. Neural stem cells constitute an ideal cell source for intracerebral transplantation and long-term drug delivery. In order to develop a cell-based system for the long-term delivery of adenosine, we isolated neural progenitor cells from adenosine kinase deficient mice (Adk(-/-)) and compared their differentiation potential and adenosine release properties with corresponding wild-type cells.

METHODS

Fetal neural progenitor cells were isolated from the brains of Adk(-/-) and C57BL/6 mice fetuses and expanded in vitro. Before and after neural differentiation, supernatants were collected and assayed for adenosine release using liquid chromatography-tandem mass spectrometry (LC-MS/MS).

RESULTS

Adk(-/-) cells secreted significantly more adenosine compared to wild-type cells at any time point of differentiation. Undifferentiated Adk(-/-) cells secreted 137+/-5 ng adenosine per 10(5) cells during 24 h in culture, compared to 11+/-1 ng released from corresponding wild-type cells. Adenosine release was maintained after differentiation as differentiated Adk(-/-) cells continued to release significantly more adenosine per 24 h (47+/-1 ng per 10(5) cells) compared to wild-type cells (3+/-0.2 ng per 10(5) cells).

CONCLUSIONS

Fetal neural progenitor cells isolated from Adk(-/-) mice--but not those from C57BL/6 mice--release amounts of adenosine considered to be of therapeutic relevance.

Purines and the Anti-Epileptic Actions of Ketogenic Diets

Ketogenic diets are high in fat and low in carbohydrates and represent a well-established and effective treatment alternative to anti-epileptic drugs. Ketogenic diets are used for the management of a variety of difficult-to-treat or intractable seizure disorders, especially pediatric refractory epilepsy. However, it has been shown that this dietary therapy can reduce seizures in people of all ages, and ketogenic diets are being applied to other prevalent medical conditions such as diabetes. Although used effectively to treat epilepsy for nearly 90 years, the mechanism(s) by which ketogenic diets work to reduce seizures remain ill-understood. One mechanism receiving increased attention is based on findings that ketogenic diets increase the brain energy molecule ATP, and may also increase the levels and actions of the related endogenous inhibitory neuromodulator adenosine. ATP and adenosine have both been identified as important modulators of seizures; seizures increase the actions of these purines, these purines regulate epileptic activity in brain, adenosine receptor antagonists are pro-convulsant, and adenosinergic mechanisms have been implicated previously in the actions of approved anti-epileptic therapeutics. Here we will review recent literature and describe findings that shed light on mechanistic relationships between ketogenic diets and the purines ATP and adenosine. These emerging mechanisms hold great promise for the effective therapeutic management of epileptic seizures and other neurological conditions.